1. Field
This invention pertains generally to nuclear reactor control rod position indication systems and more particularly to such a system that uses pulse sonar to determine the position of a control rod.
2. Related Art
Control rod position indication systems derive the axial positions of the control rods by direct measurement of the drive rod positions. Currently, both analog control rod position indication systems and digital control rod position indication systems are used in pressurized water reactors. The conventional digital control rod position indication systems have been in service for over thirty years in nuclear power stations worldwide and are currently being used as the basis for the control rod position indication systems in the new Westinghouse AP1000® designs offered by Westinghouse Electric Company LLC, Cranberry Township, Pa.
A conventional pressurized water reactor control rod position indication system is illustrated in FIG. 1 and includes a reactor vessel 10 that houses a nuclear core 28 into which control rods 30 are inserted and withdrawn to control the nuclear reaction. The control rods 30 are typically connected together by a spider and driven into and out of the core by a drive rod that moves within a pressure housing 26 in steps activated by a control rod drive mechanism (CRDM). The position of the control rods 30 relative to the core 28 are determined by rod position indicators 12.
A conventional digital rod position indication system, such as the one schematically illustrated in FIG. 1, includes two coil stacks 12 for each control rod drive rod and the associated digital rod position indication electronics 14, 20 for processing the signals from the coil stacks. Each coil stack 12 is an independent channel of coils placed over the pressure housing 26. Each channel includes twenty-one coils. The coils are interleaved and positioned at 3.75 inch (9.53 cm.) intervals (six steps). The digital rod position indication electronics for each coil stack of each control rod drive rod are located in a pair of redundant data cabinets 14A and 14B. Although intended to provide independent verification of the control rod position, conventional digital rod position indication systems are not accurate to fewer than six steps. The overall accuracy of a digital rod position indication system is considered to be accurate within plus or minus 3.75 inches (9.53 cm.) (six steps) with both channels functioning and plus or minus 7.5 inches (19.1 cm.) using a single channel (twelve steps). In contrast to conventional digital rod position indication systems, a conventional analog rod position indication system determines the position based on the amplitude of the DC output voltage of an electrical coil stack linear variable differential transformer. The overall accuracy of a properly calibrated analog rod position indication system is considered to be accurate within plus or minus 7.2 inches (18.3) (twelve steps). Neither conventional analog rod position indication systems nor conventional digital rod position indication systems are capable of determining the actual positions of the control rods.
It should be noted that for purposes of this application, the phrase “control rod” is used generally to refer to a unit for which separate axial position information is maintained, such as a group of control rods physically connected in a cluster assembly. The number of control rods varies according to the plant design. For example, a typical four-loop pressurized water reactor has fifty-three control rods. Each control rod requires its own set of coils having one or more channels and the digital rod position indication electronics associated with each channel in the case of digital systems. Thus, in a typical four-loop pressurized water reactor, the entire digital rod position indication system would include fifty-three coil stacks, each having two independent channels, and 106 digital rod position indication electronics units.
Existing control rod position indication systems use hardwired connections from the rod position detectors 12 to the rod position indication electronics cabinets 14 and from the rod position indication electronics cabinets 14 to a display cabinet 20. The analog rod position indication system employs rod position indication electronics cabinets located outside the containment 18, while the digital rod position indication system employs rod position indication electronics cabinets located inside the containment 18. During outages, fuel rods are replaced by removing the reactor vessel head. To remove the head, all rod position indicator detectors 12 must be disconnected. This can take several days depending on whether an integrated head package exists. Even in cases where an integrated head is used, the process can still take up to one full day. All cables are disconnected and manipulated, causing stress and wear on the cable assemblies. This may lead to connection issues and, ultimately, can adversely affect rod position measurement.
Currently, digital rod position indication is accomplished by detectors (69 total detector assemblies for an AP1000® plant) mounted outside and concentric with the rod travel housing 26 located above the reactor vessel head. These detectors consist of coiled wire slipped over a tube and spaced along its length at 3.75 inch (9.53 cm.) intervals. The tube fits over the drive rod travel housing. As the rod moves through the coil, the magnetic flux from the coils change. This magnetic flux change is processed by signal processing hardware to produce a measurement and reporting of rod position.
Each detector assembly requires a tube which is positioned over the rod travel housing. Also, each of the 48 detector coils per detector assembly requires several cable assemblies in order to supply the AC current needed to generate the electromagnetic field. They also need two cabinets for signal processing. Lastly, the resolution of the detectors is only plus or minus 3.125 inches (7.938 cm.) limited by the magnetic field interaction between the coils. This limitation precludes the system from achieving single-step (⅝ inch (1.59 cm.)) accuracy. Instead, the system is limited to providing positional information in groups of five steps, therefore adding un-necessary conservatism to plant safety margins.
Accordingly, a new control rod position indication system is desired that can provide greater accuracy and efficiency during refueling outages.